The technique of applying an oxide washcoat onto a ceramic flow-through honeycomb monolith in order to increase its surface area, and catalysing the product, has been very well established for some 30 years. Tens of millions of automobile catalytic convertors using this technology are made each year. Conventionally, a slurry of washcoat is deposited on the substrate, and the coated substrate is dried to form a green coated substrate which is then fired. The fired washcoated monolith is then impregnated with one or more catalytic metals from the platinum group. However, problems are beginning to emerge when trying to use this technology in more demanding situations such as with metallic substrates instead of ceramic substrates for vehicle catalytic converters, and in process catalyst systems (that is, large scale catalysed processes including sulphuric acid and nitric acid processes, steam reforming, purification processes etc). Coating durability is becoming a significant issue, especially in processes which involve the catalytic treatment of fluid systems where there are abrasive contaminants in the fluid or the catalytic device or reactor is subject to thermal and/or mechanical shocks.
It is known to use fibre reinforcement to strengthen a weaker matrix, for example in glass-fibre or carbon fibre reinforcement of plastics, and silicon carbide or nitride fibre reinforcements for metals or ceramics. Also, of course, there is reference in the Bible to making sun-dried clay bricks with straw. Most levels of fibre additions in such composites have been in the 30 to 90 wt % range. In these materials, toughness is brought about due to the strain tolerance achieved as the reinforcing fibres bridge advancing cracks in the matrix, because the failure strain of the fibres is significantly greater than the matrix. There are many ways to reinforce a matrix including, but not restricted to fibres of quartz or alumina or whiskers of SiC. There has not been, however, any suggestion of using fibres or whiskers in micron scale washcoat coatings on solid catalyst carrier substrates, and it appeared unlikely that small quantities of fibres, which may be of the same composition as at least part of the matrix washcoat material, would provide any beneficial effect. From the catalyst chemist's point of view also, there seems to be no advantage in mixing an essentially inert material into a catalyst composition.
Ahn et al (“Fabrication of silicon carbide whiskers and whisker containing composite coatings without using a metallic catalyst”, Surface and Coatings Technology, 2002, 154(2-3), 276-281) describe how composite coatings were prepared by alternative whisker growth conditions and the matrix filling process.
Zwinkels et al (“Preparation of combustion catalysts by washcoating alumina whisker-covered metal monoliths using the sol-gel method”, Studies in Surface Science and Catalysis, 1995, 91, 85-94) describe how metallic monoliths covered with alumina whiskers were dip coated in a silica containing slurry. Pd was impregnated onto the thus prepared coating to yield a suitable catalyst for catalytic combustion.
U.S. Pat. No. 5,326,735 (NE Chemcat Corp.) describes the preparation of catalyst by the deposition of iridium on a metal carbide or nitride support of which the source is unimportant but it is suggested in the patent that an inexpensive source, such as whiskers or powders with diameter 0.1 to 100 microns be used. The Ir/whisker catalyst is typically ball milled for 16 hours with binders to produce a suitable washcoat for monolith coating. No mention of enhanced coating durability is made either in the claims or the examples and it is unlikely that the whiskers survive the extensive milling process.
JP 2001252574 (Babcock-Hitachi K. K.) describes the manufacture of a catalysed fibre reinforced substrate for flue aftertreatment applications. The substrate is a multilayered structure manufactured from metal lath board spray coated with a binder such as silica sol or PVA and glass fibre non-woven cloth, which is then subsequently coated with a catalyst. In this case the fibre layer forms part of the substrate and is in effect providing a beneficial keying surface for the catalyst coating subsequently deposited.
JP 11267523 (Babcock-Hitachi K. K.) described how inorganic fibre cloth, such as silica-alumina glass fibre, can be strengthened with PVA followed by coating with a catalyst paste such as titania to furnish a catalyst coated substrate for flue glass treatment. In a further patent (JP 10202113, Babcock-Hitachi) this process is modified by sandwiching a catalyst paste between two inorganic fabric substrates and applying pressure to form strengthened catalysed articles. Both of these are examples of fibres used to manufacture composite materials which happen to be catalysed. A further example is given by JP 53015285 (Mitsubishi Heavy Industries) in which metal wire reinforced support shapes are prepared and then impregnated with catalyst precursors.
GB2138694A (Nippon Shokubai) discloses a catalyst composition which comprises a heteropolyacid-type catalytically active ingredient based on molybdophosphoric or molybdovanadophosphoric acid as a base and whiskers (whiskers typically included in an amount of 1-50% by weight based on the catalyst ingredients). The catalyst composition is reported to have excellent mechanical strengths (eg. compressive strength, abrasion resistance and falling strength) in industrial use. In one embodiment, a supported catalyst is prepared by spraying a slurry of a mixture of a compound containing a heteropolyacid as a base and whiskers onto a suitable carrier. Ordinary spherical carrier material having a diameter of 3-5 mm is exemplified. The carrier can also take the form of a solid cylinder, a hollow cylinder, broken fragments, a triangular pyramid etc, preferably at 1-10 mm, or a honeycomb or pipe.